The authors have quantitatively cross-associated the 18,811 ROSAT
All-Sky Survey Bright Source Catalog (RASS/BSC) X-ray sources with optical
sources in the USNO A-2 catalog, calculating the probability of unique
association (P_id) between each candidate within 75" of the X-ray source
position, on the basis of optical magnitude and proximity. They have
generated catalogs of RASS/BSC sources for which P_id > 98%, P_id > 90%, and
P_id > 50%, which contain 2705, 5492, and 11,301 unique USNO A-2 optical
counterparts respectively down to the stated level of significance. Together
with identifications of objects not cataloged in USNO A-2 due to their high
surface brightness (M31, M32, ...) and optical pairs, they produced a total of
11,803 associations to a probability of P_id > 50%. They also include in this
catalog a list of objects in the SIMBAD database within 10" of the USNO A-2
position, as an aid to identification and source classification. This is the
first RASS/BSC counterpart catalog which provides a probability of association
between each X-ray source and counterpart, quantifying
the certainty of each individual association. The catalog is more useful than
previous catalogs which either rely on plausibility arguments for association
or do not aid in selecting a counterpart between multiple off-band sources in
the field. Sources of high probability of association can be separated out, to
produce high-quality lists of classes (Seyfert 1/2s, QSOs, RS CVns) desired
for targeted study, or for discovering new examples of known classes (or new
classes altogether) through the spectroscopic classification of securely
identified but unclassified USNO A-2 counterparts. Low P_id associations can
be used for statistical studies and follow-on investigation - for example,
performing follow-up spectroscopy of the many low-mass stars to search for
signatures of coronal emission, or to investigate the relationship between
X-ray emission and classes of sources not previously well-studied for their
X-ray emissions (such as pulsating variable stars). The authors find that a
fraction ~65.8% of RASS/BSC sources have an identifiable optical counterpart,
down to the magnitude limit of the USNO A-2 catalog which could be identified
by their spatial proximity and high optical brightness.

This table was created by the HEASARC in October 2009 based
on electronic versions of Tables 9 (USNO counterparts for which P_id > 98%)
and 10 (USNO counterparts for which 90% > P_id > 98%) of the reference
paper which were obtained from the Astrophysical Journal web site.

This HEASARC table contains the USNO counterparts
and their nearby SIMBAD cross-identifications to the RASS/BSC sources for
which P_id > 98% (Table 9 of the reference paper),and for
which 98% > P_id > 90% (Table 10 of the reference paper), but not the lower
probability USNO counterparts (90% > P_id > 50% contained in
Table 11 of the reference paper). The expected fractions of misidentified
counterparts in these samples are 0.66%, 5.0%, and 32%, respectively.
Notice that there are many cases where there are multiple SIMBAD
cross-identifications for a single USNO A2 counterpart to a RASS/BSC X-ray
source, and that these are listed as separate rows in this table.

Name
The name of a SIMBAD Object within 10 arcseconds of the USNO
A-2 counterpart position. Notice that there are many cases where there are
multiple SIMBAD cross-identifications for a single USNO A2 counterpart to
a RASS/BSC X-ray source, and that these are listed as separate entries in
this table.

RA
The Right Ascension of the USNO A-2 counterpart to the RASS/BSC X-ray
source in the selected equinox. This was given in J2000 coordinates to an
accuracy of 0.01 seconds of time in the original reference.

Dec
The Declination of the USNO A-2 counterpart to the RASS/BSC X-ray
source in the selected equinox. This was given in J2000 coordinates to an
accuracy of 0.1 arcseconds in the original reference.

LII
The Galactic Longitude of the USNO A-2 counterpart to the RASS/BSC
X-ray source.

BII
The Galactic Latitude of the USNO A-2 counterpart to the RASS/BSC X-ray
source.

SIMBAD_Type
The SIMBAD object type of the counterpart. Notice that there
are many cases where there are
multiple SIMBAD cross-identifications for a single USNO A2 counterpart to
a RASS/BSC X-ray source, and that these are listed as separate entries in
this table. The meaning of many of the types are listed below: